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W. C. Law and S. De W. Wong
strong (negative) magnetic field, all magnetic moments within the pMTJ will align
along the same direction as the external magnetic field. However, if the (negative)
magnetic field is no longer able to favorably sustain the energy minimization along
the same direction in the SAF configuration, the weaker of the ferromagnetic section
within the SAF structure will switch first, as labelled as event ➀ occurring when
H ext = H c2 . Event ➁ occurs when H ext is now applied in the positive regime such
that H ext is greater than the coercivity of the free layer. This event will trigger the
magnetization reversal of the moments in the free layer, which were initially aligned
in the negative direction. Upon further increasing the H ext to saturation field in the
positive direction, the energy configuration is now favorable for all three magnetic
sections to be aligned in the positive field denoted as event ➂. The reverse of the
M-H loop (positive to negative H ext sweep) can be similarly explained as above. The
measurement does not retrace back on itself, due to hysteresis which is the basis
of information storage at H ext = 0 Oe. The centerline dotted across the midpoint
between HL1 and HL2 as shown in Fig. 15 is the exchange field H ex due to the
interlayer exchange coupling mediated by the thin Ru spacer layer. The exchange
coupling can be determined as J ex = H ex M s t, where M s t is the areal moment of
HL2.
In Fig. 15b, a magnetic field of smaller magnitude than the switching field of
HL2 (~600 Oe) was applied to capture the magnetic moment of only the free layer.
The M s t together with H eff determined from FMR measurements, allows for the
quantification of thermal stability of the MTJ.
6.5 Magnetic Tunnel Junction Patterning
6.5.1 Reactive Ion Etching (RIE)
The patterning process of MTJ consists of two steps; UV lithography patterning of
photoresist and subsequent etching of the SiO 2 hard mask and MTJ material. The
photoresist protects the area from being etched away. During etching, the material
is physically and chemically attacked and eroded in the unprotected areas. The etch
rate of material is a synergistic combination of both chemical and physical processes.
Vertical sidewalls and very accurate transfer of photoresist patterns to closelypacked high density of devices is feasible by anisotropic plasma etching, as wet
etching has an issue of significant undercut relative to device size. Reactive ion
etching (RIE) is done in a vacuum with a high density plasma source with a 13.6 MHz
excitation frequency and 800 W RF power. The substrate holder has a bias power
of 250–500 Wb, with an electrostatic chucking system and He backside cooling, as
shown in Fig. 16. During etching, the temperature of the wafer was kept at 70 °C
and the chamber pressure at 0.3 Pa. RIE is a combination of physical (bombardment)
and chemical (reactive) process.
W. C. Law and S. De W. Wong
strong (negative) magnetic field, all magnetic moments within the pMTJ will align
along the same direction as the external magnetic field. However, if the (negative)
magnetic field is no longer able to favorably sustain the energy minimization along
the same direction in the SAF configuration, the weaker of the ferromagnetic section
within the SAF structure will switch first, as labelled as event ➀ occurring when
H ext = H c2 . Event ➁ occurs when H ext is now applied in the positive regime such
that H ext is greater than the coercivity of the free layer. This event will trigger the
magnetization reversal of the moments in the free layer, which were initially aligned
in the negative direction. Upon further increasing the H ext to saturation field in the
positive direction, the energy configuration is now favorable for all three magnetic
sections to be aligned in the positive field denoted as event ➂. The reverse of the
M-H loop (positive to negative H ext sweep) can be similarly explained as above. The
measurement does not retrace back on itself, due to hysteresis which is the basis
of information storage at H ext = 0 Oe. The centerline dotted across the midpoint
between HL1 and HL2 as shown in Fig. 15 is the exchange field H ex due to the
interlayer exchange coupling mediated by the thin Ru spacer layer. The exchange
coupling can be determined as J ex = H ex M s t, where M s t is the areal moment of
HL2.
In Fig. 15b, a magnetic field of smaller magnitude than the switching field of
HL2 (~600 Oe) was applied to capture the magnetic moment of only the free layer.
The M s t together with H eff determined from FMR measurements, allows for the
quantification of thermal stability of the MTJ.
6.5 Magnetic Tunnel Junction Patterning
6.5.1 Reactive Ion Etching (RIE)
The patterning process of MTJ consists of two steps; UV lithography patterning of
photoresist and subsequent etching of the SiO 2 hard mask and MTJ material. The
photoresist protects the area from being etched away. During etching, the material
is physically and chemically attacked and eroded in the unprotected areas. The etch
rate of material is a synergistic combination of both chemical and physical processes.
Vertical sidewalls and very accurate transfer of photoresist patterns to closelypacked high density of devices is feasible by anisotropic plasma etching, as wet
etching has an issue of significant undercut relative to device size. Reactive ion
etching (RIE) is done in a vacuum with a high density plasma source with a 13.6 MHz
excitation frequency and 800 W RF power. The substrate holder has a bias power
of 250–500 Wb, with an electrostatic chucking system and He backside cooling, as
shown in Fig. 16. During etching, the temperature of the wafer was kept at 70 °C
and the chamber pressure at 0.3 Pa. RIE is a combination of physical (bombardment)
and chemical (reactive) process.